The present invention relates generally to lenses. In particular, the present invention is a color-corrected telecentric lens used in an optical scanning system.
Scan lenses are commonly used to focus laser beams onto an image plane in optical scanning systems. The scan lens is normally positioned between a pupil and the image plane in systems of this type. Typically, a rotating mirror located at the pupil scans the laser beams across the incident side of the lens. Because of its telecentric nature, the laser beams emerging from the lens are substantially parallel to the optical axis and therefore perpendicular to the image plane at all points along the scan.
Color scanning systems typically include three lasers for generating laser beams at three different wavelengths. The scan lens used in such a color scanning system must be capable of focusing all three laser beams onto the image plane. This design constraint imposes severe conditions upon lens designers since the optical properties of a lens are functionally related to the wavelength of light being propagated therethrough, a property known as chromatic aberration. Known color corrected scan lenses such as that disclosed in the Shibuya U.S. Pat. No. 4,396,254 often use exteme glasses with relatively low indices of refraction and high Abbe numbers (greater than 80). Glasses of this type are undesirable since they are relatively expensive, prone to striae, and often prone to straining.
There is, therefore, a continuing need for improved color-corrected telecentric scan lenses. A high performance scan lens having diffraction limited optical characteristics and a low f/ number (to provide small spot size) is desired. To be commercially viable, the lens should also be durable and relatively inexpensive to manufacture.